1. Field of the Invention
The present invention relates to a color temperature correction circuit used in an image display apparatus such as a color television receiver.
2. Description of the Related Art
In conventional color television receivers, the color temperature is set to a rather high value in order to suit the color reproduction of the white color to the viewer's taste. However, there is a problem that the reproduction of a color other than the white color becomes poor if the color temperature is set to a high value. Accordingly, various methods of correcting the color temperature only in white color parts and preventing colored parts from being corrected have been considered.
An example of the methods is described in Japanese Patent Application Laid-Open (JP-A) No. 63-160492. This example will now be described with reference to FIG. 6.
Three primary color signals R, G and B are input to subtracters 101a, 101b, 101c and 101d. The subtracters 101a, 101b, 101c and 101d output (R−G), −(R−G), (R−B) and −(R−B), respectively. The outputs (R−G) and −(R−G), and (R−B) and −(R−B) are input to maximum value detection circuits 102a and 102b, respectively. Greater one of (R−G) and −(R−G), and greater one of (R−B) and −(R−B) are selected and taken out. As a result, absolute values |R−G| and |R−B| of respective differences are obtained. Subsequently, the absolute values |R−G| and |R−B| of respective differences are compared with a reference level Vref supplied from a reference voltage generation circuit 106 in comparison circuits 103a and 103b, respectively. If the difference absolute value |R−G| or |R−B| is greater than the reference level Vref, then the pertinent comparison circuit outputs zero. If the difference absolute value |R−G| or |R−B| is less than the reference level Vref, then the pertinent comparison circuit produces an output 1 (|R−G|)or an output 2 (|R−B|)
Signal inversion circuits 103c and 103d are provided in order to obtain a positive output when the pertinent difference absolute value is lower than the reference level. Outputs of the signal inversion circuits 103c and 103d are multiplied by each other in a multiplier 104 to produce |R−G|× |R−B|as a color detection output. When R and G, and R and B are respectively at nearly equal levels, i.e. when the saturation is low, therefore, the color detection output is obtained. This indicates white to an achromatic color. A color detection characteristic in this conventional example is shown in FIG. 7. If the saturation becomes lower than a saturation corresponding to the reference level setting, the color detection output is obtained.
A gain control section 105 increases or decreases the gain of the blue color on the basis of the color detection output. As a result, white with strong blueness, i.e., a white color having a high color temperature can be obtained only at the time of an achromatic color.